This invention relates to commonly assigned Ser. No. 157,256 filed Feb. 18, 1988, now U.S. Pat. No. 4,812,491 and Ser. No. 100,537 filed Sept. 24, 1987, now U.S. Pat. No. 4,789,694 which are incorporated herein by reference. In said Ser. No. 100,537, reactive emulsion polymers treated by ion-exchange are crosslinked with glycoluril. Said Ser. No. 157,256 pertains to self-curing ion-exchange treated latexes containing copolymerized monomers including carboxyl and hydroxyl monomers in combination with alkylol acrylamide monomer to provide a self-curing thermosetting latex.
This invention relates to protective surface coatings and more particularly to paint coatings containing a thermosetting polymeric binder based on an ion-exchange treated, thermosetting functional latex polymer. The functional latex polymer comprises emulsion polymerized ethylenic monomers including a beta hydroxy ester monomer, but specifically excluding amine monomers, to produce a functional latex. The functional latex is then treated in an ion-exchange process to produce a low pH functional latex adapted to self-crosslink by transesterification with respective beta-hydroxy ester groups in the latex polymer.
With respect to prior art, several patents disclose the use of acids, bases, metal salts, and organic metal complexes as catalysts for transesterifying polymers such as U.S. Pat. Nos. 4,362,847; 4,376,848; 4,332,7811; and U.S. Pat. No. 4,459,393 wherein octoates or naphthenates of lead, zinc, calcium, barium, and iron are disclosed as transesterification catalysts.
Dante and Parry have shown that phosphonium halides, such as ethyltriphenyl phosphonium iodide, are efficient catalysts for (a) 1,2-epoxide reactions with phenols to yield hydroxyl ethers (U.S. Pat. No. 3,477,990), and (b) polyepoxide reactions with carboxylic acids or acid anhydrides (U.S. Pat. No. 3,547,885). The patents suggest that polyepoxides and phenols can be reacted to form phenolic hydroxyl ethers with phosphonium salts as catalysts. The counterion of the phosphonium moiety is the anion portion of a carboxylic acid, or acid ester, such as in ethyltriphenyl phosphonium acetate (U.S. Pat. No. 3,948,855).
Barnhoorn et al (U.S. Pat. No. 4,459,393) teach self-crosslinking thermosetting resin compositions obtained from the reaction of a beta-hydroxylalkyl ester of an alpha, beta-carboxylic acid with a primary mono- or polyamine to give a product having 1 to 2 amino hydrogens and further reacted with a polyglycidyl ether of a polyhydric phenol so that the final resin adduct has more than one betahydroxyalkyl ester group and amine groups having 1 to 2 amine hydrogen atoms per molecule. Transesterification catalysts known in the art are taught.
Subamanyam et al (U.S. Pat. No. 4,376,848) teach the preparation of water dilutable electrocoating compositions having tertiary amino-containing basic binders by reacting a secondary amino group compound with an olefinically doublebonded epoxy and the copolymerization of this product with at least one ethylenically bonded polymerizable monomer wherein said binders can self-cure and be cured in combination with amine resins and/or phenolic resins. Common transesterification catalysts are taught.
Velko patents disclose resinous compositions curable through a transesterification curing mechanism based on conventional heavy metal catalysts. For instance, U.S. Pat. No. 4,423,167 discloses a polymeric polyol adapted to be crosslinked with a polyester having at least two beta-alkoxyester groups in the presence of conventional transesterification catalysts. Similarly, U.S. Pat. No. 4,489,182 is based on a crosslinking agent having at least two delta-hydroxy ester groups, while U.S. Pat. No. 4,423,169 is based on a crosslinking agent at least two beta- and/or gammaester groups, and U.S. Pat. No. 4,423,168 is based on a crosslinking agent having at least two beta-amide ester groups. The Velko patents utilize as catalysts organic salts of conventional heavy metal catalysts such as lead, zinc, iron, tin and manganese.
It now has been found that substantially odor free, ambient room temperature or heat-cured thermosetting consumer latex paints can be produced and cured by the internal reaction of beta-hydroxy ester functional latex binders. In accordance with this invention, the functional latex is ion exchanged to obtain a low pH, preferably below 2.5, to provide an acid activated thermosetting latex that can be heat cured or ambient cured without external crosslinking agents. It has been found that ambient dry consumer trade sales latex paint compositions (wall and ceiling paints, etc.) can be formulated to produce highly desired air dry films which further crosslink over a period of time to produce substantially improved film integrity properties. Ambient room temperature ordinarily is considered to be about 25.degree. C. plus/minus 50.degree. C., although higher/lower application temperatures are encountered with consumer paints. Prior to this invention, conventional air drying consumer paints dried by evaporation of water and subsequent coalescence of binder polymer particles, but such binders were merely coalesced and to crosslinked. In accordance with this invention, the binder polymer particles coalesce but then further crosslink at ambient room temperature or heat cured without the need for conventional crosslinkers such as melamine, glycoluril or methylol amide monomers in the latex. Elimination of melamine crosslinkers for instance substantially reduces or eliminates undesirable formaldehyde emissions. Instead, the functional latex polymer of this invention can be applied at room temperature, air dried for a few days to remove water, and then become crosslinked. Room temperature cured consumer paints exhibit excellent film formation by air drying but subsequently crosslink at ambient room temperature to form a cured thermoset paint film exhibiting superior film integrity properties.
In accordance with the process of this invention, an ambient or moderately heat curable latex paint free of melamine or glycoluril-type crosslinkers can be produced by emulsion copolymerization of ethylenic monomers using conventional surfactants and subjecting the resulting latex polymer to a cationic exchange treatment to remove the cations from the anionic surfactant as well as from other sources. The emulsion binder polymer is substantially free of amine groups since amine groups block reactivity, inhibit the cure and render the ionic exchange process of this invention inoperative. The resulting cationic exchange step produces an emulsion polymer adapted to cure at room temperature without the addition of acid catalysts such a p-toluenesulfonic acid. Hence, the disadvantages associated with the use of external acid catalysts are overcome by this invention. It is believed that the ambient cure achieved through ion exchange provides a low pH (acidic) system which activates crosslinking upon evaporation of water from the applied paint film. The thermosetting reaction occurring during the transesterification cure of the beta-hydroxy addition polymer (latex) is an acidic catalyzed coreaction of a beta-hydroxy ester group with another betahydroxy ester group. This invention is based on introducing acid catalysis into the polymeric mixture by ion-exchanging the beta-hydroxy ester latex polymer until the pH thereof is very acidic and preferably below 2.5 which avoids the addition of undesirable external acid catalysts.
Pigmented or clear paint mixtures exhibit excellent uninhibited cure along with long-term viscosity stability. The ion-exchange process utilizes a proton-substituted cation exchange resin to remove cations and unexpectedly yields an ambient cure liquid coating with long term package stability. Package stability is also improved, due to the absence of acid-sensitive formaldehyde crosslinking compounds. The cation exchange treatment of the thermosetting latex provides stability against settling and flocculation. The improved clear or pigmented emulsion coatings of this invention are indefinitely stable at room temperature and, upon drying, thermoset at room temperature to give a fully crosslinked film. These and other advantages of the invention will become more apparent by referring to the specification and illustrative examples.